Sheet-fed, offset printing machines comprise a rotating plate cylinder that carries a printing plate to which is fed a supply of ink. The printing portions of the printing plate are inked by the supplied ink to thereby form an inked image on the printing plate. The plate cylinder engages a rotating rubber-blanket cylinder, or blanket cylinder, and transfers the inked image from the printing plate onto the rubber blanket disposed thereon. A sheet to be printed is fed to the printing machine between the blanket cylinder and a rotating impression cylinder. The inked image then is transferred from the rotating blanket cylinder onto the printed sheet.
The blanket cylinder carries a rubber blanket over only a portion of its circumferential surface. To retain the rubber blanket on the surface of the blanket cylinder, the blanket cylinder is provided with an axial channel in which means for retaining the rubber blanket are disposed. The useful printing portion of the blanket cylinder thus is bounded by a print start edge and a print end edge of the rubber blanket, the print start edge being that edge that leads during the rotation of said rotating cylinder with respect to the rubber blanket.
During the printing process, printing ink residue may remain on the blanket cylinder, or may be transferred to the impression cylinder. In addition, residue from the printed sheets, such as paper fluff, may settle onto the blanket cylinder and may be retained thereon. These print residues may impair the quality of the image printed by the printing machine, particularly when a new print order is run using a new printing plate. It is therefore necessary to periodically clean the blanket and impression cylinders.
The prior art has taught to provide a washing roller for cleaning the blanket or impression cylinders, the washing roller fluidically communicating with a source of washing fluid. When the washing roller is used to clean the blanket or impression cylinder, the surface of the washing roller is wetted with washing fluid to thereby provide a washing surface. The washing roller is movable between a cleaning position in which the washing surface engages the rotating blanket or impression cylinder and an idle position in which the washing surface does not engage the rotating blanket or impression cylinder. To clean one of the cylinders, for example, the blanket cylinder, the washing roller first is moved to its cleaning position. The washing roller then is rotated in a direction opposite the direction of rotation of the rotating blanket cylinder to thereby clean the rubber blanket.
This method allows the blanket or impression cylinder to be cleaned quickly. It has been observed, however, that many print residues still remain on the rotating cylinder after it is cleaned, particularly in the case of the blanket cylinder. Specifically, the print end edge of the rubber blanket is not cleaned completely by the washing roller, inasmuch as the washing roller tends to deposit the print residue onto the print end edge as it runs off the rubber blanket into the axial channel of the blanket cylinder. It is believed that this deposit may contribute to soiling of the printing sheets in the next print order. Although this deposit of print residue may be reduced by increasing the duration of the cleaning cycle, it is difficult to remove these deposits completely, and the increase in duration of the washing cycle is time consuming and uneconomical.
Accordingly, a need exists for a method for cleaning a rotating cylinder in a printing machine in which print residue deposits are reduced or eliminated, particularly at the print end edge of a rubber-blanket cylinder. It is a general object of the present invention to provide such a method for cleaning a rotating cylinder in a printing machine.